Start pep8 protection.

Introduce a pep8 check on the files {edited,added} in the current commit.
For now, this will force @vecna to fix every pep8 violation in any file he
edits. Otherwise, the build will result broken.
Remove scripts/build-glbackend.sh - #1046
Remove scripts/build-testing-package.sh - #1046

In the parent commit, build-glbackend has been removed. Therefore, this script
is of no more use now.
Remove scripts/build-glclient.sh - #1046

This script is no more working. Plus, it doesn't contain anything that the
GlobaLeaks developer isn't able to do by hand easily.